View Full Version : "telephoto" large format lens
I have noticed that some lens have a "tele" or "telephoto" designation. Are these lens designed to be used on cameras with short bellows draw, ie... a 360mm lens on a camera with a 300mm maximum bellow extension.
By definition, a telephoto lense is any lense with a focal length that is longer than a so called "normal" focal length lense. In large format cameras the focal length of a particular lense requires approximately the same amount of bellows draw to focus at infinity as the focal length of the lense. i.e. a 360mm lense r equires approximately that amount of bellows draw. When you see a "tele" or "tel ephoto" designation on a large format lense, however, what they are really sayin g is that you can get that focal length equivalent without the customary amount of bellows draw. I know that Nikon makes several, and I know of at least one fro m Schneider. What happens is that you can get an 1800mm focal length and only ne ed 600mm of bellows draw. Most cameras that I'm aware of don't have anywhere nea r 1800mm of bellows draw. Most 8X10 cameras have up to about 600mm, several 4X5 cameras have excellent bellows draw, but usually less than 600mm. So you can see that the longest focal length lense you could use that is of normal constructio n would be a 600mm (focused at infinity). That is why these companies have desig ned the "tele" lenses. Be careful, however, because as I said, telephoto can mea n two different things. On one hand it means longer than normal focal length, on the other hand it can mean that the bellows focusing distance is less than the focal length. If you have any more questions regarding this, feel free to e-mail me.
True tele designs in large format have to do with the placement of the nodal poi nt. I am not an optical engineer, so won't try to explain it. Most of the larg e format texts discuss the scientific stuff. Tele lenses for large format allow short bellows draw cameras to be used for fie ld work with longer focal lengths. The old Calument catalogues discussed these lenses. ( Likewise, the old Zone VI catalogues.) I work with a Wista 45DX. The max bellows draw is 300 mm. In conventional desi gns the longest lens I could use ( and then at near infininty focus ) is a 240. With tele designs, I could use the 360 and longer lengths for near infinity foc us. (It is a way for us sheet film nuts to reach out and get personal with a di stant subject, just like the roll film shooters.)
Typically, I believe that most telephotos for any format size is a magnification of about 1.5 or 1.6. In other words about half again or 50% more magnificatio n. Half again more than 12 inches is 18 inches. Therefore an 18 inch tele woul d probably focus at about 12 inches. Of course these are infinity focus figures , as all lens measurments are listed as the focal length at infinity focus.
What you are all talking about is called effective focal length. This is used by designers in all formats to accomplish special needs. When the distance between the front element and the film plane is either shorter or longer than the desig nated focal length on the lens, this indicates the use of effective focal length . Medium format SLRs that need a wide angle lens that is removed some distance f rom the film plane, need to have a lens that has an effective focal length that is short, while the distance from the front element to the film plane is much gr eater. Telephotos on the other hand like to have the effective focal length long er than the lens itself due to size, weight or draw limitations. It should be re membered that not all high to moderate magnification lenses are telephotos. Only the ones that use a negative element or group to change the focal length are tr ue telephotos. Steve
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